Modifications of different zeolite types are described in "Chemie in unserer Zeit" (Chemistry in Our Time), volume 20, 1986, No. 4, page 121. The objective of such modifications is to change the catalytic and adsorptive properties, as well as to increase the thermal, chemical and hydrothermal stability of the zeolite structure. For the improvement of "shape-selective" properties, very expensive methods frequently are required, such as the incorporation of phosphorus compounds into the zeolite lattice or the synthesis of metal complexes in the cavities of the zeolite framework. Shape-selectivity refers to the capacity of a catalyst to allow: (1) only one type or reactant molecule to pass through the pores of the catalyst to reach the catalytic sites (or sorption sites); and (2) only product molecules formed at the catalytic sites having predetermined dimensions, to diffuse out of the pores of the catalyst. Thus, shape-selective catalysts are useful in increasing the selectivity of a catalyst to a specific, desired product. Such catalysts are used in several important reactions, including for example: the conversion of methanol to gasoline; the conversion of methanol to olefins; the production of ethylbenzene from benzene and ethene; xylol isomerization processes; and several organic syntheses important in the preparation of organic intermediates.
From various investigations it is known for example that, for the disproportionation of ethylbenzene, the product selectivity (for paradiethylbenzene, based on the isomeric diethylbenzene formed), which represents a measure of the shape-selective properties of the zeolite catalyst, lies between 30 and 40% for a 2% conversion for ZSM-5 zeolite EQU ((Na,TPA).sub.3 [(A1O.sub.2).sub.3 (SiO.sub.2).sub.93 ].multidot.16H.sub.2 O)
prepared with tetrapropylammonium (TPA) and between 40 and 80% for ZSM-5 that is prepared template-free according to the present invention, for crystallites of the same size and with comparable Si:Al ratios (Dissertation by U. Girrbach, University of Mainz, Germany 1987). The term "template-free" refers to the preparation of the ZSM-5 zeolite without TPA or other organic compounds.